An ionization chamber

ABSTRACT

The invention relates to an ionization chamber ( 10 ) comprising an inner spherical electrode ( 2 ), an outer spherical electrode ( 4 ), a space between the inner spherical electrode and the outer spherical electrode, and a resistive hollow body ( 3 ) provided in the said space, wherein electrical connections to the inner spherical electrode and electrical connection to the top of the resistive hollow body ( 3 ) are electrostatically shielded by that same resistive hollow body having a continuously varied local resistance along its axis. The invention further relates to a method of manufacturing an ionization chamber.

FIELD OF THE INVENTION

The invention relates to an ionization chamber, in particular theinvention relates to a spherical ionization chamber.

The invention further relates to a method of manufacturing an ionizationchamber.

BACKGROUND OF THE INVENTION

Spherical ionization chambers are known per se. For example, CN101526622 describes a detector device for radiation monitoring, whichcomprises an ionization chamber part comprising a shell and an electrodepart, a circuit part for processing electrical signals from theelectrode part, and a metal seal box, wherein the circuit part isarranged in the metal seal box. The known spherical ionization chamberis filled with a gas in a volume between the inner electrode and theouter electrode.

Such spherical ionization chambers may be used for dosimetry purposesfor enabling measurements without directional sensitivity. However, itis found that such spherical ionization chambers do demonstratedirectional sensitivity in strong external magnetic fields, such as themagnetic fields present in a magnetic resonance apparatus, cyclotron orfusion reactor.

In particular, it appears that the electrical connection, necessary forproviding voltage to the inner spherical electrode, distorts theotherwise spherical electrical field distribution between the innerelectrode and the outer electrode.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a spherical ionizationchamber for use in relatively strong magnetic fields, such as those of amagnetic resonance apparatus, wherein distortion of the E-field isavoided or is substantially mitigated.

To this end an ionization chamber according to the invention comprisesan inner spherical electrode, an outer spherical electrode, a spacebetween the inner spherical electrode and the outer spherical electrode,and a resistive hollow body provided in the said space, whereinelectrical connections to the inner spherical electrode and electricalconnection to the top of a resistive hollow body are electrostaticallyscreened by that same resistive hollow body having a continuously variedlocal resistance along its axis.

It will be appreciated that by way of its functioning the sphericalionization chamber always comprises a necessary electrical connection tothe inner electrode.

It is found that by providing a resistive body extending substantiallyradially between the inner surface of the outer spherical electrode andthe inner spherical electrode, thereby screening the electricalconnection to the inner electrode, the distortion of the E-field causedby said connection may be mitigated or substantially overcome. Thecontinuously varied local resistance of the resistive body is adapted tomimic the spatial electrostatic potential characteristics that would bepresent between the charged inner spherical electrode and the outerspherical electrode. In this way the resulting ionization chamber ishighly immune to magnetic fields.

To achieve this, it is clear that the electrical resistance shoulddepend on the radius r along the resistive body. Usually such resistivebody is relatively thin, i.e. comparable with he dimensions of theelectrical wire used to enable electrical connection to the innerelectrode. In order to derive how, we consider the system of chargedconductors only, because this is exactly what has to be “mimicked” bythe resistive body.

This simplifies the system to an inner charged small sphere with aradius ‘a’ at an electric potential V_(a), being located at the centreof a larger hollow conducting sphere with inner radius ‘b’.

For a<r<b the associated electrical potential

${V(r)} = {V_{a} \cdot R_{a} \cdot {\left( \frac{1 - \frac{R_{b}}{r}}{R_{a} - R_{b}} \right).}}$

Accordingly, at a given r the local resistance of the resistive layeraround a thin central pole should be inverse proportional to thedistance from the centre. The electrical potential at point r along thepole can be regarded as the wiper of a potentiometer.

The power dissipation in the resistive pole should be limited. With atypical driving voltage of 1000V and a resistive pole of total 10⁷ Ω thepole current would be limited to 100 micro ampere and the dissipatedpower to 100 mW.

Any material to manufacture high voltage electrical resistive componentsaccording to the present art can be used. Some non-limiting examplesare: carbon, metal films or slightly conductive polymers. The desireddistance dependent resistance can e.g. be achieved by varying the layerthickness inversely to the distance.

It will be appreciated that those skilled in the art readily appreciatehow to calculate necessary values of the varied resistance for sphericalionization chambers having different geometrical and electriccharacteristics.

An embodiment of a spherical ionization chamber is known from Kim H. S.et al “Performance of a high-pressure xenon ionization chamber forenvironmental radiation monitoring”, Radiation Measurements, Elsevier,Amsterdam, N L, vol. 43, no. 2-6, 1 Feb. 2008, pages 659-663.

The known ionization chamber describes an inner spherical electrode andan outer spherical electrode. However, a shielding mesh known used inthe known ionization chamber, being a per se known Frisch grid, isintended to be operable at a single and constant potential and ismanufactured from a well electrically conductive material. Accordingly,the grid shall be always at the same potential in use at a given time.In addition, it will be appreciated that the Frisch mesh is manufacturedfor allowing a passage of a flux of charged particles through theopenings in the mesh.

It is a disadvantage of the known spherical ionization chamber that theFrisch grid, being operable at the constant potential value, introducesundesirable perturbations into the electric field in the inner volume ofthe ionization chamber.

In the invention, however, the resistive hollow body provided in theinner space of the spherical ionization has a variable potential alongits length. Accordingly, the surrounding resistive body neutralizes anyperturbation of the electric field inside the spherical ionizationchamber which may occur due to the internal conductor.

In addition, it is noted that the resistive hollow body in the sphericalionization chamber of the invention may be advantageously designed to beembodied by a cylindrical or tapered hollow resistor, so that onepotential along the length of the resistor varies from the value presetat the inner electrically conductive electrode to the potential valuepreset at the outer electrically conductive electrode. In this way theresistive body shields the inner high voltage electrode. Accordingly,charged particles in the gas volume of the ionization chamber will notbe influenced by the presence of the high voltage lead field, becausethis field is effectively shielded by the resistive body.

In an embodiment of the ionization chamber according to the inventionthe resistive body has a larger dimension at its base portion on theouter spherical electrode and a smaller dimension at its top near theinner spherical electrode. Preferably, the dimension of the base isabout 5 mm and the dimension of the top is about 1 mm. Due to the factthat the resistive body is adapted to restore the spherical symmetry ofthe E-field between the inner electrode and the outer electrode, a valueof the resistance at the top of the resistive body is larger than avalue of the resistance and the base of the resistive body.

In a further embodiment of the ionization chamber according to theinvention, the top of the resistive body is connected to the innerspherical electrode by an insulator.

It is found to be advantageous to provide a relatively stiff connectionbetween the top portion of the resistive body and the inner electrodefor preserving radial orientation of the resistive body inside theionization chamber.

A method for manufacturing an ionization chamber according to theinvention comprises the steps of:

-   -   providing an inner spherical electrode, an outer spherical        electrode, a space between the inner spherical electrode and the        outer spherical electrode and a hollow resistive body in said        space;    -   screening an electrical connection to the inner spherical        electrode and to the top of the hollow resistive body using the        same hollow resistive body having a continuously varied local        resistance, wherein said resistive body is radially arranged in        said space between an inner surface of the outer spherical        electrode and the inner spherical electrode.

Further advantageous embodiments of the method according to theinvention are set forth in claims 8-11.

These and other aspects of the invention will be discussed in moredetail with reference to the figures, wherein like reference signsrelate to like elements.

BRIEF DESCRIPTION

FIG. 1 presents in a schematic way an embodiment of a cross-section ofthe spherical ionization chamber according to the invention.

FIG. 2 presents in a schematic way an embodiment of a cut-away of thespherical ionization chamber according to the invention.

FIG. 3 presents in a schematic way an embodiment of the resistive bodyaccording to the invention.

DETAILED DESCRIPTION

FIG. 1 presents in a schematic way an embodiment of a cross-section ofthe spherical ionization chamber according to the invention. Theionization chamber 10 is a so-called spherical ionization chamber,wherein the inner spherical electrode 2, having the radius r_(a) isconcentrically arranged with respect to the outer spherical electrode 4having the respective radius r_(b). The inner spherical electrode 2 isconnected by suitable wires 5, 6 to a voltage source 8. The lead wires5, 6 carry the same potential but are different in function. The wire 5provides the potential for the hollow resistive body 3 for enablingcorrection of the E-field. The wire 6 carries the ionization currentwhich is to be measured. The top portion of the hollow resistive body 3must be electrically isolated from the inner spherical electrode 2. Theoutside of the resistive body 3 has to be electrically isolated forpreventing any space charge from leaking through the body 3. The hollowresistive body 3 may be manufactured from a limited number of individualresistors, from a slightly conductive material formed to a correct shapefor obtaining a continuous variation in local resistance, or from anon-conductive pole covered with different purposefully provided layersof conductive material for obtaining a structure having asemi-continuous change in local resistance. The potential from thevoltage source 8 is provided to the outer spherical electrode usingconnection 4 a.

In accordance with the invention a hollow resistive body 3 is providedbetween the inner surface of the outer spherical electrode 4 and theinner spherical electrode 2. The resistive body 3 is adapted with asuitable set of variable resistances for screening the wires 5, 6 andfor maintaining the radial potential between the inner electrode 2 andthe outer electrode 4 which corresponds to an unperturbed situation. Thewire 6 may comprise an amperemeter for retrieving an electrical signalcharacterizing the ionization within the chamber 10 during use. Moredetails on the hollow resistive body 3 will be discussed with referenceto FIG. 3.

The hollow resistive body 3 is electrically connected to the outerspherical electrode 4 at a base portion 9 of the body 3. The hollowresistive body 3 may be mechanically connected to the inner electrode 2by means of a support member I using a highly isolating material formaintaining the radial orientation of the resistive body 2. It will beappreciated that the support member I is electrically isolated from boththe inner electrode and wire 5 connecting the top of the resistive bodyto the voltage source 8.

It is also appreciated that the outside of the resistive body iselectrically insulated from the environment.

The electrical resistance in the resistive body in according to theinvention depends on the radius r along the resistive body. In order toderive the suitable local values of the resistances a system of chargedconductors may be considered, because this is exactly what has to bemimicked by the resistor body 3.

This simplifies the system to an inner charged small sphere with aradius ‘a’ carrying a charge ‘q’, being located at the centre of alarger hollow conducting sphere with inner radius ‘b’.

The electric potential for this system of nested spheres is given by:

$\begin{matrix}{{V(r)} = {V_{a} - {\frac{V_{a} - V_{b}}{\frac{1}{R_{b}} - \frac{1}{R_{a}}} \cdot \left( {\frac{1}{r} - \frac{1}{R_{a}}} \right)}}} & (1)\end{matrix}$

As the absolute potential is arbitrary, one may set V_(b)=0V so that:

$\begin{matrix}{{V(r)} = {V_{a} \cdot R_{a} \cdot {\left( \frac{1 - \frac{R_{b}}{r}}{R_{a} - R_{b}} \right).}}} & (2)\end{matrix}$

Accordingly, at a given r the local resistance of the resistive layeraround the thin central body 3 should be inversely proportional to thedistance from the centre. The electrical potential at point r along thepole can be regarded as the wiper of a potentiometer.

The resulting resistive ladder has to reproduce this potential at thenodes between the resistors. In en embodiment where the resistive poleis comprised of a finite number of N resistors, the voltage at the nodeN is given by

$\begin{matrix}{V_{j} = {\frac{\sum\limits_{k = 1}^{j}\; R_{k}}{\sum\limits_{k = 1}^{N}\; R_{k}} \cdot {V_{a}.}}} & (3)\end{matrix}$

The combination of (2) and (3) gives (N−1) non-trivial equations for theN unknown values of R_(j). So the solution to that set of linearequations gives the relative values of all resistors. To obtain absolutevalues for all resistors and extra equation has to be added that setsthe total current through the resistors.

The power dissipation in the resistive pole should be limited. With atypical driving voltage of 1000V and a resistive pole of total 10⁷ Ω thepole current would be limited to 100 micro ampere and the dissipatedpower to 100 mW.

The physical size of the resistors that is chosen determines the maximumvalues of N. Of course, the larger N, the better the approximation is ofthe node voltages to the real potential. Another approximation arrivesfrom the limited choice of commercially available resistance values. Forexample for r_(a)=2.5 mm and r_(b)=25 mm and a total resistance of about10⁷ Ω, the following commonly available resistance values may be used:

R1=107 kOhm

R2=124 kOhm

R3=150 kOhm

R5=243 kOhm

R6=332 kOhm

R7=464 kOhm

R8=715 kOhm

R9=1 MOhm

R10=6 MOhm.

In another embodiment one may use layers of resistive coating instead ofphysical resistors. The layer thickness would change as a function of rin order to obtain the correct potential at every location r. Anymaterial to manufacture high voltage electrical resistive componentsaccording to the present art can be used. Some non-limiting examplesare: Carbon, metal films or slightly conductive polymers.

FIG. 2 presents in a schematic way an embodiment of a cut-away of thespherical ionization chamber according to the invention. In this viewonly a lower half 4 a of the outer ionization chamber is shown. Theresistive body 3 is connected at its base portion B to the inner surfaceof the outer ionization chamber 4 a. The top portion T of the resistivebody 3 is provided adjacent the inner electrode (not shown for clarity).

In accordance with the invention the resistive body 3 is provided with aseries of individual resistors 3 a, . . . 3 n, wherein the value of aresistor arranged at the top portion is continuously decreasing to alower value 3 n.

It is also possible to provide a variation in dimension of the resistivebody for achieving the result of a continuously varying resistance. Forexample, a cross-sectional dimension of the resistive body 3 at the topportion T may be larger than a cross-sectional dimension of theresistive body at the base portion. In this case the resistive body maybe manufactured from the same material, as the difference in the localresistance will be attributed to a local difference in a volume of thematerial. A suitable material for manufacturing the resistive body iscarbon, metal film or a slightly conductive polymer.

FIG. 3 presents in a schematic way an embodiment of the resistive body 3according to the invention. It this embodiment the resistive body 3comprises portions of the individual resistances 3 a, 3 b, 3 c, 3 d, 3e, 3 f, 3 g, 3 h, 3 i, 3 j. It will be appreciated, however, that adifferent number of the resistive elements may be used. The localresistance 3 a at the top portion of the resistive body is larger thatthe local resistance 3 j at the base portion of the resistive body 3.Preferably, the values of the individual resistances depend inverseproportionally to the distance r from the centre of the ionizationchamber.

While specific embodiments have been described above, it will beappreciated that the invention may be practiced otherwise than asdescribed. The descriptions above are intended to be illustrative, notlimiting. Thus, it will be apparent to one skilled in the art thatmodifications may be made to the invention as described in the foregoingwithout departing from the scope of the claims set out below.

1. An ionization chamber comprising an inner spherical electrode, anouter spherical electrode, a space between the inner spherical electrodeand the outer spherical electrode, and a resistive hollow body, having atop and a base, provided in the space between the inner sphericalelectrode and the outer spherical electrode, wherein electricalconnections to the inner spherical electrode and electrical connectionto the top of a resistive hollow body are electrostatically screened bythat same resistive hollow body having a continuously varied localresistance along its axis.
 2. The ionization chamber according to claim1, wherein the resistive body has a tapered rod shape.
 3. The ionizationchamber according to claim 1, wherein the continuously varied localresistance is adapted to enable a substantially undisturbed radialpotential between the inner spherical electrode and the outer sphericalelectrode.
 4. The ionization chamber according to claim 3, wherein avalue of the local resistance at the top is larger than a value of thelocal resistance at the base of the resistive body.
 5. The ionizationchamber according to claim 1, wherein the top of the resistive body isconnected to the inner spherical electrode by an insulator.
 6. Theionization chamber according to claim 1, wherein the resistive bodyextends substantially radially between the outer electrode and the innerelectrode.
 7. A method for manufacturing an ionization chamber,comprising the steps of: providing an inner spherical electrode, anouter spherical electrode, a space between the inner spherical electrodeand the outer spherical electrode and a hollow resistive body, theresistive body having a top and a base portion, in said space; screeningan electrical connection to the inner spherical electrode and to the topof the hollow resistive body using the same hollow resistive body havinga continuously varied local resistance, wherein said resistive body isradially arranged in said space between an inner surface of the outerspherical electrode and the inner spherical electrode.
 8. The methodaccording to claim 7, wherein the resistive body has a larger dimensionat its base portion on the outer spherical electrode and a smallerdimension at its top near the inner spherical electrode.
 9. The methodaccording to claim 7, wherein the continuously varied local resistanceis adapted to enable a substantially radial equipotential distributionbetween the inner spherical electrode and the outer spherical electrode.10. The method according to claim 9, wherein a value of the resistanceat the top is larger than a value of the resistance at the base of theresistive body.
 11. The method according to claim 7, wherein the top ofthe resistive body is connected to the inner spherical electrode by aninsulator.